1. FIELD OF THE INVENTION
The invention relates to improvements in the manufacture of sulfhydryl-terminated compounds useful as chemical intermediates and precursors of polythioether elastomeric sealants. By the method of the invention a substantially colorless sulfhydryl-terminated product of improved purity is obtained at an increased rate in the reaction of hydrogen sulfide and an allene compound. Subsequent polymerization of the crude product with a polyunsaturated olefin and/or an acetylene provides a substantially colorless sulfhydryl-terminated polythioether of the type in demand as a base polymer for sealants.
2. DESCRIPTION OF THE PRIOR ART
The ultraviolet light-initiated free-radical liquid phase addition of hydrogen sulfide to allene compounds (i.e. compounds containing the allene linkage, &lt;C.dbd.C.dbd.C&gt;) is known to produce sulfhydryl-terminated compounds i.e. dithiols and sulfhydryl-terminated thioethers. U.S. Pat. No. 3,488,270, to Griesbaum et al., discloses the liquid phase addition of hydrogen sulfide to allene initiated with ultraviolet light. The reaction is conducted in quartz which transmits high energy ultraviolet light, i.e. ultraviolet light of wavelength below 2600 Angstroms, as well as low energy, long wavelength ultraviolet light, i.e., ultraviolet light of wavelength of about 2600 to 3800 Angstroms. The crude reaction mixture is yellow and is usually contaminated with elemental sulfur. The formation of sulfur is accompanied by hydrogen gas evolution which may produce sufficient pressure to burst the reaction vessel. It has not been found feasible by conventional purification techniques, e.g. distillation, to remove the discoloration. This disadvantage is particularly serious since the principal reaction product, 1,3-propane dithiol, is (via polymerization with an unsaturated hydrocarbon) the precursor of sulfhydryl-terminated polythioethers which cure to valuable architectural sealants. Thus, when the discolored dithiol is reacted with an acetylenically and/or polyethylenically unsaturated compound in the presence of ultraviolet light as initiator according to known techniques (see U.S. Pat. Nos. 3,592,798 and 3,717,618 to Oswald and coassigned U.S. Ser. No. 501,716, filed Aug. 29, 1974, entitled "Polythioether Sealant Compositions" to B. F. Dannels), the sulfhydryl-terminated polythioether obtained is also discolored, and of unsatisfactory curing characteristics and, hence, is commercially unacceptable. Moreover the propanedithiol product must be recovered e.g. by fractional distillation, of the crude reaction mixture, if polymerization of the dithiol with the unsaturate is to be achieved. Apparently in the prior art allene-hydrogen sulfide reaction a side product is formed which inhibits the polymerization. In addition to the propanedithiol the crude reaction mixture contains about 30-50 weight percent sulfhydryl-terminated hydrocarbons with one, two, three, four or more thioether linkages, conveniently designated sulfhydryl-terminated thioether oligomers. These thioethers, because of the presence of terminal sulfhydryl substituents, would reasonably be expected to condense with the unsaturated reactant in the polymerization. However, because of the necessity to recover the propane dithiol from the crude reaction mass these sulfhydryl-thioether reactants are unavoidably lost, i.e. because of their relatively high boiling points, they cannot be readily recovered by distillation for use in the polymerization.
In coassigned U.S. Ser. No. 535,631, now U.S. Pat. No. 3,992,289 entitled "Improved Method for Production of Sulfhydryl Compounds" to B. F. Dannels and E. J. Geering, filed of even date herewith, an improved method of reacting an allene compound, such as allene, and hydrogen sulfide is disclosed whereby the problem of yellow color and sulfur and hydrogen formation associated with the prior art reaction of an allene and hydrogen sulfide is overcome. According to the latter application, the pertinent technology of which is incorporated herein by reference, initiation of the reaction with ultraviolet light of wavelength substantially above about 2600 Angstroms, provides a substantially colorless reaction mixture free of sulfur. However, in order to obtain a dithiol polymerizable with the unsaturated reactant, the reaction mixture of the latter improved process must also be subjected to a costly, tedious dithiol recovery step which involves loss of the valuable sulfhydryl-terminated thioether oligomers.
U.S. Pat. Nos. 2,398,481 and 2,411,983 to W. E. Vaughan et al. and U.S. Pat. No. 2,873,239 to W. R. Nummy et al. disclose liquid phase addition of hydrogen sulfide to unsaturated olefins in the presence of ultraviolet light of wavelength above 2900 Angstroms and of photosensitizing compounds. The reference processes, however, do not relate to reaction of olefins having the distinctive allene unsaturated group, i.e. two carbon-to-carbon double bonds attached to the same carbon atom and, hence, do not serve to remedy the problems encountered in the aforementioned prior art reaction of an allene and hydrogen sulfide.